1. Field of the Invention
The present invention relates generally to mixers and in particular to a mixer that exhibits very good linearity at low voltages.
2. Description of the Related Art
A mixer is a circuit that receives separate input signals and combines the signals to produce an output signal. Such circuit is an important building block in communications, for example, wireless radio frequency devices. Mixers may be also widely used in infrared networks and fiber optic communication systems. Generally, mixers, may be used where frequency up or down conversion may be required in a modulation scheme. Frequency conversion is the changing of one frequency to another. This may occur in instances when one signal is multiplied with a second signal to produce, a sum and/or difference of the signals. By using this feature, in one example, the mixer allows very high frequency to be downconverted to baseband or intermediate frequency (IF) so that signals may be evaluated using analog or digital signal processing techniques. In another example, the mixer upconverts a low frequency to a very high frequency.
One well-known mixer configuration is the Gilbert cell which is illustrated in FIG. 1. The configuration of the Gilbert cell may be two differential bipolar transistor pairs P1,P2 cascaded with an output of another differential bipolar transistor pair P3 to form a four-quadrant mixer. A four-quadrant mixer allows input signals to be combined for all polarities of signals. In comparison, a two-quadrant mixer allows input signals to be combined for the same polarity of signals. For practical purposes, a mixer should at least be a two-quadrant mixer. Using bipolar transistors in the Gilbert cell may be advantageous in that bipolar transistors are exponential law devices. Stated differently, a relationship between collector to base voltage V.sub.CB and the collector current I.sub.C is exponential. Although the relationship may not be purely linear in all regions, it is approximately linear in certain regions of the relationship. The equation below expresses the relationship between the differential output current and the two input voltages V.sub.1 and V.sub.2 of the Gilbert cell. ##EQU1##
I.sub.EE is the bias current and V.sub.T is the threshold voltage. Note that the hyperbolic tangent function is an infinite series representing multiple harmonics of the input signals and the output signals as expressed by the equation below. ##EQU2##
Typically, a bandpass filter passes the desired output signal and filters out the remaining undesirable signals. While the Gilbert cell may be successfully implemented in circuits using bipolar transistors, such may not be the case where Gilbert cell configuration is used in a complementary metal oxide semiconductor (CMOS) circuit.
FIG. 2 illustrates a Gilbert cell configuration using CMOS devices. One disadvantage of CMOS is that it is a square law device. Stated differently, drain current I.sub.D is a square law function of gate to source voltage V.sub.GS, therefore, non-linear distortion, inherently exist when the mixer is driven by input signals using gate to source or gate to drain relationships. The equation below expresses the output differential current in terms of input voltages V.sub.1 and V.sub.2. ##EQU3##
I.sub.DSS is the bias current and V.sub.p is the pinch-off voltage. From the equation above, it may be readily understood that a square law device is inherently nonlinear.
Another disadvantage that exists regarding Gilbert cell using CMOS devices is that the Gilbert cell design is based on stacked differential pairs, (i.e. one differential pair is stacked on top of another differential pair). Stated differently, a supply voltage may need to be sufficient to sustain the collector to emitter voltages VCE of two stacked transistors and corresponding loads. A stacked differential pair is generally not a problem in bipolar transistor circuits where it is presumed to operate at high voltage supply, for example, 6-12 volts. However, CMOS circuits are generally designed to operate at lower voltages, typically, 3 volts or lower. With the advancement of integrated circuit technology, CMOS circuits may be expected to operate at 2 volts or lower. This complicates circuits using a stacking arrangement, for example, the Gilbert cell, since it generally requires voltage to be higher than 3 volts to drive stacked transistors. Therefore, a Gilbert cell using CMOS devices may not be compatible for usage in low voltage circuits, for example, portable communication devices such as cellular phones.
Another problem with the Gilbert cell design is that the bi-polar transistor being an exponential law device or, in the case of CMOS device, a square law device, the output produces multiple harmonics requiring unwanted signals to be filtered out using a filter. This requires additional circuits and results in consumption of valuable power which is a problem in portable communication devices. In response, the present invention is directed to a mixer that does not use stacked differential pairs like the Gilbert cell. Instead, the mixer uses parallel differential pairs that may allow the mixer to operate at supply voltages of 1.2 volts or lower, for example. Further, the mixer overcomes the non-linear distortion inherent in CMOS devices.